The present invention relates to voltage controlled oscillators, and more particularly to a multi-octave, wideband voltage controlled oscillator suitable for use in instruments that require a broad range of frequencies, such as spectrum analyzers, frequency synthesizers, sweepers or the like.
For many uses a broadband range of frequencies is desirable. Conventionally spectrum analyzers, as one of the uses, have used magnetically tunable filters and oscillators that use ferrite resonators, such as Yttrium-Iron-Garnet or other materials. One such YIG oscillator is shown in U.S. Pat. No. 4,827,230. This is a rather large, heavy and power hungry oscillator. A bank of voltage controlled oscillators has been used to provide a broadband range, each oscillator having a voltage signal output with the outputs being switched to a load using radio frequency (RF) switches to select the desired frequency range from the bank, as shown in U.S. Pat. No. 3,921,085. At least one octave of frequency is desired, and continuous coverage over multiple frequency octaves is preferred. However switching the outputs of the oscillators in the bank does not necessarily mean turning off the oscillator itself. Therefore additional circuitry is needed to also turn off the oscillators that are not selected in order to save power. Also when providing two frequency outputsxe2x80x94one for a main output and another for a phase locked loop (PLL) reference frequency outputxe2x80x94the power output is split between the two outputs through the use of a power divider network.
Also very high frequency oscillators are desired to be used, especially as the communications frequencies increase to 10 GHz and above. One such high frequency voltage controlled oscillator is shown in U.S. Pat. No. 5,418,500. However the output, which is at the second harmonic frequency of the oscillator fundamental frequency, is a low power output referenced to a xe2x80x9cbalanced neutral point.xe2x80x9d Also since the fundamental frequency is taken directly from the tank circuit, the tank circuit is loaded down by the load which reduces the Q of the circuit, increasing the phase noise. Small changes, such as noise, on the control or tuning signal may result in large phase noise due to crosstalk. Also the coupling capacitors are part of the tank circuit which reduces the effect of the varactor since it is desired that the variable capacitance be the largest part of the tank circuit.
Another proposed high frequency voltage controlled oscillator that provides a low power output has a pair of inductors coupled to ground with the opposing ends being coupled to respective bases of a pair of transistors. A pair of opposing varactors are coupled in series across the emitters, forming with the base-emitter and emitterxe2x80x94emitter capacitances a Colpitts oscillator. A tuning control signal is applied at the junction of the varactors. However having the varactors at the emitters of the transistors changes the loop gain, which is undesirable.
What is desired is a multi-octave, wideband voltage controlled oscillator suitable for various broad range frequency applications that uses minimal power, i.e., is suitable for battery operation, while maintaining low phase noise.
Accordingly the present invention provides a multi-octave, wideband voltage controlled oscillator in the form of a bank of high frequency voltage controlled oscillators that provide a current output. The individual oscillators are selectable on command by turning on the desired oscillator(s), the others remaining off. The outputs are wire-OR""d together so that the selected output is available to respective limiter/divider circuits without loss of power. A main output from the main limiter/divider provides the desired frequency octave, and a phase locked loop (PLL) output from the reference limiter/divider provides a PLL reference frequency. Each individual oscillator has a tank circuit consisting of a lumped inductance, or a length of transmission line, in parallel with a pair of opposed varactors coupled in series with a voltage control signal being applied between virtual ground at the lumped inductance and the junction of the varactors. The tank circuit is coupled across the bases of a pair of emitter-coupled transistors in a balanced common collector Colpitts oscillator configuration, with the current output being taken from the collectors. A summer/subtractor circuit may be added at the output of each VCO or at the input of the limiter/dividers to select either a fundamental or a second harmonic frequency from the selected VCO.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.